Routing: RIP, OSPF, Hierarchical routing, BGP

Transcription

1 Advanced Networking Routing: RIP, OSPF, Hierarchical routing, BGP Renato Lo Cigno Routing Algorithms: One or Many? Is there a single routing protocol in the Internet? How can different protocols and algorithms coexist Homogeneous results Risk of incosistent routing Complexity of routing algorithms/protocols Can they scale? There is a tradeoff between traffic and computation? Hierarchical routing Policy routing: what is it, why not performance? Advanced Networking Routing 2 RIP - History * Late 1960s: Distance Vector protocols were used in the ARPANET * Mid-1970s: XNS (Xerox Network system) routing protocol is the precursor of RIP in IP (and Novell s IPX RIP and Apple s routing protocol) * 1982: Release of routing software for BSD Unix * 1988: RIPv1 (RFC 1058) - classful routing * 1993: RIPv2 (RFC 1388) - adds subnet masks with each route entry - allows classless routing * 1998: Current version of RIPv2 (RFC 2453) Advanced Networking Routing 3 1

2 RIP at a glance A simple intradomain protocol Straightforward implementation of Distance Vector Routing Distributed version of Bellman-Ford (DBF) with well known issues slow convergence works with limited network size Strengths simple to implement simple management widespread use Advanced Networking Routing 4 RIP at a glance Metric based on hop count maximum hop count is 15, with 16 equal to imposed to limit the convergence time the network administrator can also assign values higher than 1 to a single hop Each router advertises its distance vector every 30 seconds (or whenever its routing table changes) to all of its neighbors RIP uses UDP, port 520, for sending messages Changes are propagated across network Routes are timeout (set to 16) after 3 minutes if they are not updated Advanced Networking Routing 5 Recall: counting to infinity problem B 1 1 C D NTW_1 A Router A NTW_1 D 2 Router B NTW_1 A 3 Router C NTW_1 A 3 Router D Consider the entries in each routing table for network NTW_1 Router D is directly connected to NTW_1 NTW_1 dir 1 Advanced Networking Routing 6 2

3 Recall: counting to infinity problem (2) C B A time Router A NTW_1 Unr. - Router B Router A NTW_1 C 4 Router B Router A NTW_1 C 5 Router B 10 D 1 NTW_1 A 3 Router C NTW_1 C 4 Router C NTW_1 C 5 Router C NTW_1 Link between B and D fails NTW_1 A 3 Router D NTW_1 dir 1 NTW_1 B 4 Router D NTW_1 dir 1 NTW_1 B 5 Router D NTW_1 dir 1 Advanced Networking Routing 7 Recall: counting to infinity problem (3) B 1 1 C 1 10 D A time Router A NTW_1 C 11 Router B NTW_1 C 11 Router C Router A NTW_1 C 12 Router B NTW_1 C 12 Router C NTW_1 NTW_1 B 11 Router D NTW_1 D 11 Router D NTW_1 dir 1 NTW_1 dir 1 Advanced Networking Routing 8 RIP: solution to counting to infinity Maximum number of hops bounded to 15 this limits the convergence time Split Horizon simple each node omits routes learned from one neighbor in update sent to that neighbor with poisoned reverse each node include routes learned from one neighbor in update sent to that neighbor, setting their metrics to infinity drawback: routing message size greater than simple Split Horizon Advanced Networking Routing 9 3

4 RIP: solution to counting to infinity (cont d) Triggered updates: nodes send messages as soon as they notice a change in their routing tables only routes that has changed are sent faster reaction but more resources are used (bandwidth, processing) cascade of triggered updates superposition with regular updates Advanced Networking Routing 10 Command: 1=request 2=response Updates are replies whether asked for or not Initializing node broadcasts request Requests are replied to immediately Version: 1 Address family: 2 for IP IP address: non-zero network portion, zero host portion Identifies particular network Metric Path distance from this router to network Typically 1, so metric is hop count RIP-1: Message Format one route entry (20 bytes) IP UDP RIP Message 32 bits Command Version 0 0 address family 0 0 IP address (32-bit) metric address family 0 0 IP address (32-bit) metric (up to 25 total route entries) Advanced Networking Routing 11 RIP procedures: introduction RIP routing tables are managed by application-level process e.g., routed on UNIX machines Advertisements are sent in UDP packets (port 520) RIP maintains 3 different timers to support its operations Periodic update timer (25-30 sec) used to sent out update messages Invalid timer (180 sec) If update for a particular entry is not received for 180 sec, route is invalidated Garbage collection timer (120 sec) An invalid route in marked, not immediately deleted For next 120 s. the router advertises this route with distance infinity TCP IP Data Link Physical route UDP Advanced Networking Routing 12 4

5 RIP procedures: input processing Request Messages they may arrive from routers which have just come up action: the router responds directly to the requestor s address and port request is processed entry by entry Response Messages they may arrive from routers that perform regular updates, triggered updates or respond to a specific query action: the router updates its routing table in case of new route or changed routes, the router starts a triggered update procedure Advanced Networking Routing 13 RIP procedures: output processing Output are generated when the router comes up in the network if required by the input processing procedures by regular routing update Action: the router generates the messages according to the commands received the messages contain entries from the routing table timers timers timers input output input output input output request response response response response Advanced Networking Routing 14 Version: 2 Route Tag: used to carry information from other routing protocols e.g., autonomous system number Subnet mask for IP address Next hop identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0.0) RIPv2: Message Format one route entry (20 bytes) IP UDP RIPv2 Message Command Version 0 0 address family address family 32 bits IP address (32-bit) Subnet mask Next hop metric IP address (32-bit) Subnet mask Next hop metric Route Tag Route Tag (up to 25 total route entries) Advanced Networking Routing 15 5

6 Any host sending packets on UDP port 520 would be considered a router Malicious users can inject fake routing entries With authentication, only authorized router can send Rip packets Authentication type password MD5 Authentication plain text password MD5 hash RIPv2: authentication authentication entry route entry IP UDP RIPv2 Message Command Version 0 0 0xFFFF address family 32 bits Authentication IP address (32-bit) Subnet mask Next hop metric Authentication Type Route Tag (up to 24 total route entries) Advanced Networking Routing 16 RIPv2: other aspects Explicit use of subnets Interoperability RIPv1 and RIPv2 can be present in the same network since RIPv1 simply ignores fields not known RIPv2 responds to RIPv1 Request with a RIPv1 Response Multicast instead of broadcasting RIP messages, RIPv2 uses multicast address Advanced Networking Routing 17 RIP limitations: the cost of simplicity Destinations with metric more than 15 are unreachable If larger metric allowed, convergence becomes lengthy Simple metric leads to sub-optimal routing tables Packets sent over slower links Accept RIP updates from any device (if no security is implemented) Misconfigured device can disrupt entire configuration Advanced Networking Routing 18 6

7 RIP Was the first... but... Why is RIP not enough to manage the Internet? Can Link-State protocols perform better? OSPF MOSPF (no MRIP exists!!) Inter-AS routing requires an entirely different approach... if not for else for the sake of competition! Advanced Networking Routing 19 Non-RIP, DV Protocols: EXAMPLE IGRP (Interior Gateway Routing Protocol) CISCO proprietary; builds on RIP (mid 80 s) Distance Vector, like RIP several cost metrics (delay, bandwidth, reliability, load etc.) uses TCP to exchange routing updates routing tables exchanged only when costs change Loop free routing achieved by using a Distributed Updating Alg. (DUAL) based on diffused computation In DUAL, after a distance increase, the routing table is frozen until all affected nodes have learned of the change (cfr. split horizon in RIP) Advanced Networking Routing 20 Open Shortest Path First (OSPF) RIP limited in large internets OSPF is often preferred interior routing protocol for TCP/IP based internets Uses link state routing Floods the messages to all routers in the AS (area) Advanced Networking Routing 21 7

8 OSPF advanced features (not in RIP) Security: all OSPF messages are authenticated (to prevent malicious intrusion); TCP or Unicast in genera connections used sometimes Multiple same-cost paths allowed only one path in RIP For each link, multiple cost metrics for different TOS (eg, satellite link cost set low for best effort; high for real time) Integrated uni- and multicast support: Multicast (MOSPF) uses same topology data base as OSPF Hierarchical OSPF in large domains Advanced Networking Routing 22 Link State Routing When initialized, router determines link cost on each interface Router advertises these costs to all other routers in topology Router monitors its costs When changes occurs, costs are re-advertised Each router constructs topology and calculates shortest path to each destination network No distributed version of routing algorithm Can use any algorithm Dijkstra is recommended and normally used All routers in AS must use same algorithm Advanced Networking Routing 23 Flooding Packet sent by source router to every neighbor Incoming packet resent to all outgoing links except source link Duplicate packets already transmitted are discarded Prevent incessant retransmission All possible routes tried so packet will get through if route exists Highly robust At least one packet follows minimum delay route Reach all routers quickly All nodes connected to source are visited All routers get information to build routing table High traffic load Advanced Networking Routing 24 8

9 Flooding Example Advanced Networking Routing 25 Alternative to flooding Designated Router (DR) election (with backup-drb) Used on broadcast domains Link-State updates are sent to DR/DRB only which diffuse to all others (unicast confirmed communications) destinations DR destinations destinations Advanced Networking Routing 26 OSPF Overview Router maintains descriptions of state of local links Transmits updated state information to all routers it knows about (flooding) Router receiving update must acknowledge Lots of traffic generated Each router maintains database Directed graph Advanced Networking Routing 27 9

10 Router Database Graph Vertices Router Network Transit Stub Edges Connecting two routers Connecting router to network Built using link state information from other routers Advanced Networking Routing 28 Sample Autonomous System Advanced Networking Routing 29 Directed Graph of Autonomous System in previous slide Advanced Networking Routing 30 10

11 Link Costs Cost of each hop in each direction is called routing metric OSPF provides flexible metric scheme based on type of service (TOS) Normal (TOS) 0 Minimize monetary cost (TOS 2) Maximize reliability (TOS 4) Maximize throughput (TOS 8) Minimize delay (TOS 16) Each router can generate 5 spanning trees (and 5 routing tables) AS decision! Advanced Networking Routing 31 What is the SP for Router 6? Advanced Networking Routing 32 The Tree for Router R6 Advanced Networking Routing 33 11

12 OSPF Packet Header Advanced Networking Routing 34 Packet Format Notes Version number: 2 is current Type: one of 5, see next slide Packet length: in octets including header Router id: this packet s source, 32 bit Area id: Area to which source router belongs Authentication type: Null Simple password Encryption Authentication data: used by authentication procedure Advanced Networking Routing Hello: used in neighbor discovery OSPF Packet Types 2. Database description: Defines set of link state information present in each router s database 3. Link state request 4. Link state update 5. Link state acknowledgement Advanced Networking Routing 36 12

13 Areas Make large internets more manageable Configure as backbone and multiple areas Area Collection of contiguous networks and hosts plus routers connected to any included network Backbone contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas Advanced Networking Routing 37 Hierarchical OSPF Advanced Networking Routing 38 Operation of Areas Each area runs a separate copy of the link state algorithm Topological database and graph of just that area Link state information broadcast to other routers in area Reduces traffic Intra-area routing relies solely on local link state information Advanced Networking Routing 39 13

14 Path consists of three legs Within source area Intra-area Through backbone Has properties of an area Inter-Area Routing Uses link state routing algorithm for interarea routing Within destination area Intra-area Advanced Networking Routing 40 Hierarchical OSPF Two level hierarchy: local area and backbone Link state advertisements do not leave respective areas Nodes in each area have detailed area topology; they only know direction (shortest path) to networks in other areas Area Border routers summarize distances to networks in the area and advertise them to other Area Border routers Backbone routers run an OSPF routing alg limited to the backbone Boundary routers connect to other ASs Advanced Networking Routing 41 Intra-AS and Inter-AS routing a C.b b C d A A.a a b A.c c B.a a B c Gateways: perform inter-as routing amongst themselves b perform intra-as routers with other routers in their AS inter-as, intra-as routing in gateway A.c network layer link layer physical layer Advanced Networking Routing 42 14

15 Intra-AS and Inter-AS routing a Host h1 C C.b b A.a Inter-AS routing between A and B A.c a d A b c Intra-AS routing within AS A B.a a B c b Host h2 Intra-AS routing within AS B We ll examine specific inter-as and intra-as Internet routing protocols shortly Advanced Networking Routing 43 Inter-AS routing BGP (Border Gateway Protocol): the de facto standard Path Vector protocol an extension of Distance Vector Each Border Gateway broadcast to neighbors (peers) the entire path (ie, sequence of AS s) to destination For example, Gwy X may store the following path to destination Z: Path (X,Z) = X,Y1,Y2,Y3,,Z Advanced Networking Routing 44 Inter-AS routing Now, suppose Gwy X send its path to peer Gwy W Gwy W may or may not select the path offered by Gwy X, because of cost, policy or loop prevention reasons If Gwy W selects the path advertised by Gwy X, then: Path (W,Z) = w, Path (X,Z) Note: path selection based not so much on cost (eg,# of AS hops), but mostly on administrative and policy issues (eg, do not route packets of competitor s AS) Advanced Networking Routing 45 15

16 Why different Intra- and Inter-AS routing? Policy: Inter is concerned with policies (which provider we must select/avoid, etc). Intra is contained in a single organization, so, no policy decisions necessary Scale: Inter provides an extra level of routing table size and routing update traffic reduction above the Intra layer Performance: Intra is focused on performance metrics; needs to keep costs low. In Inter it is difficult to propagate performance metrics efficiently (latency, privacy etc). Besides, policy related information is more meaningful. We need BOTH! Advanced Networking Routing 46 Border Gateway Protocol (BGP) Allows routers (gateways) in different ASs to exchange routing information Messages sent over TCP Messages in next slide Three functional procedures Neighbor acquisition Neighbor reachability Network reachability Advanced Networking Routing 47 BGP Messages Open Start neighbor relationship with another router Update Transmit information about single route List multiple routes to be withdrawn Keepalive Acknowledge open message Periodically confirm neighbor relationship Notification Send when error condition detected Used for closing connections too Advanced Networking Routing 48 16

17 Neighbor Acquisition Neighbors attach to same subnetwork If in different ASs routers may wish to exchange information Neighbor acquisition is when two neighboring routers agree to exchange routing information regularly Needed because one router may not wish to take part One router sends request, the other acknowledges Knowledge of existence of other routers and need to exchange information established at configuration time or by active intervention Advanced Networking Routing 49 Neighbor Reachability Periodic issue of keepalive messages Between all routers that are neighbors Each router keeps database of subnetworks it can reach and preferred route When change is made, router issues update message (to neighbors only) All BGP routers build up and maintain routing information Advanced Networking Routing 50 BGP Message Formats Advanced Networking Routing 51 17

18 Neighbor Acquisition Detail Router opens TCP connection with neighbor Sends open message Identifies sender s AS and gives IP address Includes Hold Time As proposed by sender If recipient prepared to open neighbor relationship Calculate hold time min [own hold time, received hold time] Max time between keepalive/update messages Reply with keepalive Advanced Networking Routing 52 Keepalive Detail Header only Enough to prevent hold time expiring If hold time expires a topology change is triggered Marker is a field that used for authentication purposes Advanced Networking Routing 53 Update Detail Information about single route through internet Information to be added to database of any recipient router Network layer reachability information (NLRI) List of network portions of IP addresses of subnets reached by this route Total path attributes length field Path attributes field (next slide) List of previously advertised routes being withdrawn May contain both Advanced Networking Routing 54 18

19 Path Attributes Field Origin Interior (e.g. OSPF) or exterior (BGP) protocol AS_Path ASs traversed for this route Next_Hop IP address of boarder router for next hop Multi_Exit_disc Information about routers internal to AS Local_Pref Tell other routers within AS degree of preference Atomic_Aggregate, Aggregator Uses subnet addresses in tree view of network to reduce information needed in NLRI Advanced Networking Routing 55 Withdrawal of Route(s) Route identified by IP address of destination subnetwork(s) May be issued because subnets are not reachable or because policies have changed Advanced Networking Routing 56 Notification Message Error notification Message header error Includes authentication and syntax errors Open message error Syntax errors and option not recognised Proposed hold time unacceptable Update message error Syntax and validity errors Hold time expired Finite state machine error Cease Close connection in absence of any other error Advanced Networking Routing 57 19

20 BGP Routing Information Exchange R1 constructs routing table for AS1 using OSPF R1 issues update message to R5 (in AS2) AS_Path: identity of AS1 Next_Hop: IP address of R1 NLRI: List of all subnets in AS1 Suppose R5 has neighbor relationship with R9 in AS3 R5 forwards information from R1 to R9 in update message AS_Path: list of ids {AS2,AS1} Next_Hop: IP address of R5 NLRI: All subnets in AS1 R9 decides if this is prefered route and forwards to neighbors Advanced Networking Routing 58 Routing Domain Confederations Set of connected AS Appear to outside world as single AS Recursive Effective scaling Advanced Networking Routing 59 20